“Millions of Americans are tossing perfectly good food in the trash because they think it’s not safe to eat after the date on the package," Dana Gunters, a senior scientist with the National Resources Defense Council, said in a statement last year.

Americans throw away up to 40% of purchased food every year, the equivalent of $218 billion. In addition to the wasted money — about $1,500 per year for a family of four, according to the NRDC — food waste also means that all of the resources that were used to grow, store, and transport food get wasted, too.

Here's how to tell if everything in your refrigerator — including milk, cheese, hummus, and fish — has gone bad.

A bad egg floats.

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Egg shells are slightly porous, and as they age, small sacs of air begin to form between the shell wall and the egg.

Fresh fish should be eaten within 36 hours of purchase, according to Whole Foods, EatByDate, and The Kitchn.

Fish that's spoiled will typically have a slimy flesh with a thick, slippery coating. It may also develop a fishier-than-normal smell.

Fresh vegetables turn yellow when they're going bad.

Green vegetables turn yellow when they expire.

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That said, some vegetables can still be salvageable, according to the Greater Chicago Food Depository. They suggest trimming the discolored portion of veggies like celery and soaking it for 10-15 minutes in ice water to refresh.

That's because bread is porous, meaning that mold spreads easily throughout the loaf.

Bead that gets stale, hard, or dry, on the other hand, isn't necessarily expired. Mold requires moisture to grow, so dry bread may still be perfectly fine to eat .It can typically be safely used to make bread crumbs or croutons.

If mold grows on soft cheeses like cottage or cream cheese, they should be discarded, according to the Mayo clinic. That's also true for shredded or sliced cheese.

Harder cheeses, however, take longer to spoil because of their density. In many cases, it's possible to cut off a moldy outer layer and find the interior to be fine, since mold doesn't penetrate far into harder cheeses like cheddar or parmesan.